Sometimes a room thermostat is combined with a time switch. A time switch is an electrical switching device operated by a clock to provide one or more on periods for the space heating or domestic (hot water) heating system. These are called programmable room thermostats. There are three basic types of programmable room thermostats: (a) the standard programmer, (b) the full programmer, and (c) the mini-programmer.
The standard programmer type controls both the space heating and domestic hot-water heating with the same time settings. A full programmer thermostat, on the other hand, provides independent time settings for space heating and domestic hot-water heating. This allows the two to operate independently of one another. Finally, the mini-programmer permits the domestic hot-water heating to be on alone (without space heating) or to be on together with the space heating. It does not allow the space heating to be on alone.
Some room thermostats have a night setback feature, which reduces energy use by lowering temperatures at night when the occupants are sleeping. These are called day-night (or twin-type) thermostats. They comprise an assembly of two thermostats mounted on a single base operating in conjunction with a timer or clock. The electric clock can be set to throw the temperature controls from one thermostat for the daytime onto the other for the night (or vice versa) at a predetermined time setting on the clock. This conveniently permits a low temperature at night and normal temperature during the day. Figure 4-29 shows a wiring diagram of a typical twin-type thermostat and illustrates the connections between the clock and primary control.
Day-night thermostats designed to provide automatic temperature switching control for only a heating system can be modified to provide system and fan switching. The Honeywell T882 Chronotherm clock thermostat provides these functions for a heating and cooling installation when used with the Honeywell Q611A thermostat subbase (see Figures 4-30 and 4-31).